A drum unit of a conventional color printer includes a pair of side plates supporting a plurality of photosensitive drums therebetween. Each rear end portion of each side plate is formed with a notched portion. Further, a front beam constituting a front end portion of the drum unit is inserted with a support shaft. The main casing has metal plates, and a base shaft is spanned between rear end portions of the metal plates. Each metal plate has a front end portion formed with a notched groove. The notched portion of the drum unit is engageable with the base shaft, and each end portion of the support shaft is engageable with each notched groove. Thus, the front and rear end portions of the drum unit can be positioned at a fixed position relative to the main casing.
Further, each side plates of the drum unit has a lower end portion provided with a downwardly protruding portion so as to prevent each photosensitive drum from being in direct contact with a desk when the drum unit is removed from the main casing and placed on the desk.